I am current reading the Mixing Engineers Handbook, fricking sweet. I have a question about compression. In the book the author keeps making statements like the following.

"Hit the compressor fairly hard, at least 10db or more if it sounds good"

"When controlling dynamics, usually a very small amount of compression(2db to 4db or so at a 2:1 to 4:1 ratio) is used to limit the peaks of a signal"

In the two quotes above I bolded the parts that I don't understand. I know what threshold, attack and release, and makeup gain are but I don't know what it means to measure compression in decibels. To what control does this refer?

Thanks

Brett